Drill rods are rods that usually have a male thread in one end and a female thread in the other end. A common drill rod may be 6 m long but may extend to up to ca 10 meters. A number of drill rods, that have been jointed by screwing the male thread in a drill rod together with the female thread in the adjacent drill rod in thread joint, form a drill string whose length commonly is 50 meters long, up to 100 meters. One end of the drill string culminates in a drill bit which drills into the rock when it strikes and rotates. The other end of the drill rod is jointed to a driving device via a shank adapter. At operation the drill string strikes and is rotated by means of the driving device that is arranged on a drill rig from which the drill string projects. When almost all of the drill string is drilled into the rock, the drill string is lengthened with one drill rod at a time by slacking the drill string from the shank adapter and screwing another drill rod on the other end of the drill string and the shank adapter.
At drilling with high torques on the drill string a strong tightening of the thread joints of the drill string is often obtained. To be able to slacken these thread joints, the striking apparatus is allowed to continue to strike on the drill string in axial direction after drilling of the hole has been completed to shake loose the first thread joint to be slackened, i.e. the thread joint that joints the shank adapter with its closest drill rod. This leads to large loadings on drill rig, feeding beam, drill and drill steel. In addition is not sure that the drill rod that is closest to the shank adapter slackens off, but that another rod joint that is inside the rock. There is then a risk that parts of the drill string are lost, especially if the drilling takes place downwards the rock and the drill string is affected by the gravitation. This is a time consuming process that requires a large torque in back rotation in order for the drill string to slacken off.
WO0201041 discloses a method to slacken joints in drill steel in a drilling device. At slacking the pressure from a striking device on the drill steels and the joint receives a gentle axial impact as mentioned above. The drill steel is held simultaneously as it is rotated.
WO 03/033858 shows a striking device with a striking head, which striking device is adapted to reduce torque between two jointed drill rods at slacking off. The device is manually turned to the drill rods to be slackened and manually damped around these by an operator. The device clamps around the jointed drill rods from two diametrically different directions. This is done by a transferring arm on the striking device pressing on the jointed drill rods against the very striking head on the striking device by means of a coil spring. The striking head, i.e. one of the clamping points then strikes with radial strikes against or close to the joint to decrease the tensions that result due to deformations in the joint. The drill steels are then relatively rotated and thereby detached. This is a complicated device that must be manually handled. In addition it is difficult to achieve stability since one of the clamp points also is strike point, which means that the clamping power decreases when the striking head is on its way back and until a new strike comes. This may also imply a risk that parts of the drill string are dropped, especially if the drilling takes place downwards the rock and is affected by the gravity.